1. Field of the Invention
The present invention relates to a sound control apparatus for a keyboard-based musical instrument, which is applied to an electronic keyboard-based musical instrument such as an electronic piano and a composite piano such as a silent piano and an automatic playing piano for setting a sound generation timing.
2. Description of the Prior Art
A known conventional sound control apparatus for a keyboard-based musical instrument is disclosed, for example, in Laid-open Japanese Patent Application No. 2-160292. This sound control apparatus 61 is applied to an upright automatic playing piano, and as illustrated in FIG. 1, comprises a swingable key (not shown), a hammer 63 for pivotal movement about a center pin 68 in association with a touch on the key to strike a string 62, a shutter 64 attached to the hammer 63, a first and a second sensor 65, 66, and the like. The shutter 64 is formed in an arcuate shape which has one end fixed to a front surface of a hammer shank 63a, and the other end fixed on a top surface of a catcher 63b, respectively. The shutter 64 is also formed with an arcuate shutter window 67 conformal thereto. This shutter window 67 comprises an upper half 67a and a lower half 67b which is offset toward the hammer shank 63a, i.e., the rear side with respect to the upper half 67a. 
The first and second sensors 65, 66 are arranged adjacent to each other at positions corresponding to the upper half 67a and lower half 67b of the shutter window 67. Each of the sensors 65, 66 comprises one set of a light emitter and a light receiver (none of which is shown) disposed on one and the other sides of the shutter 64.
With the foregoing configuration, light from the light emitter of the first sensor 65 is intercepted by the shutter 64, while light from the light emitter of the second sensor 66 reaches the light receiver through the lower half 67b of the shutter window 67 in a key released state indicated by solid lines in FIG. 1. From the key released state, the shutter 64 pivotally moves together with the hammer 63, to the accompaniment of pivotal movement of the hammer 63 in the counter-clockwise direction in FIG. 1, associated with a touch on the key. Associated with this pivotal movement, the rear end of the upper half 67a of the shutter window 67 through the shutter 64 reaches the first sensor 65, causing light to reach its light receiver. As the hammer 63 further pivotally moves, a leading edge of the lower half 67b of the shutter window 67 passes the second sensor 66, and intercepts light from the light emitter of the second sensor 66. As the hammer 63 further pivotally moves, the leading edge of the upper half 67a of the shutter window 67 passes the first sensor 65 immediately before the hammer 63 strikes the string 62, thereby intercepting the light from the light emitter of the first sensor 65. On the other hand, when the key is released, detection signals of the first and second sensors 65, 66 change in the order reverse to the foregoing.
In this sound control apparatus 61, a timing at which the detection signal of the second sensor 66 indicates a light path closed state and the detection signal of the first sensor 65 switches from an open state to a closed state is set and recorded as a sound generation timing at which sound should be generated in a automatic play. Also, a timing at which the detection signal of the second sensor 66 indicates an open state and the detection signal of the first sensor 65 switches from the open state to the closed state is set and recorded as a sound stop timing. After striking the string 62, the hammer 63 pivotally moves in the clockwise direction in FIG. 1, to return to its home position, and in the halfway, the catcher 63b abuts to a back check 69 implanted on a wippen (not shown) and stops.
However, in the conventional sound control apparatus 61, since the shutter 64 is attached to the catcher 63b, the shutter 64 tends to come into contact with the back check 69, making the hammer 63 more susceptible to rebound. As the hammer 63 rebounds in this way, the shutter 64 can close the light paths of the second sensor 66 and first sensor 65 in this order. In this event, the first and second sensors 65, 66 generate the same detection signals as those which are generated at the sound generation timing, resulting in erroneous generation of sound, though no key touch operation is actually performed.
In addition, the shutter 64 comes into contact with the back check 69 to cause vibrations which are transmitted to the key through an associated action, thus impairing a touch feeling. Further, since the sound stop timing is set in the manner described above, the shutter 64 must be attached such that it closes the light path of the first sensor 65 and opens the light path of the second sensor 66 in the key released state. Such assembling work requires much labor and time.